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<body class="manpage">
<div id="header">
<h1>
zmq_poller(3) Manual Page
</h1>
<h2>NAME</h2>
<div class="sectionbody">
<p>zmq_poller -
   input/output multiplexing
</p>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="paragraph"><p><strong>void *zmq_poller_new (void);</strong>
<strong>int zmq_poller_destroy (void *</strong><em>poller_p</em>);*</p></div>
<div class="paragraph"><p><strong>int zmq_poller_add (void *<em>poller</em>, void *<em>socket</em>, void *<em>user_data</em>, short <em>events</em>);
*int zmq_poller_modify (void *<em>poller</em>, void *<em>socket</em>, short <em>events</em>);</strong>
<strong>int zmq_poller_remove (void *<em>poller</em>, void *<em>socket</em>);</strong></p></div>
<div class="paragraph"><p><strong>int zmq_poller_add_fd (void *<em>poller</em>, int <em>fd</em>, void *<em>user_data</em>, short <em>events</em>);</strong>
<strong>int zmq_poller_modify_fd (void *<em>poller</em>, int <em>fd</em>, short <em>events</em>);</strong>
<strong>int zmq_poller_remove_fd (void *<em>poller</em>, int <em>fd</em>);</strong></p></div>
<div class="paragraph"><p><strong>int zmq_poller_wait (void *<em>poller</em>,
                          zmq_poller_event_t *<em>event</em>,
                          long <em>timeout</em>);</strong>
<strong>int zmq_poller_wait_all (void *<em>poller</em>,
                          zmq_poller_event_t *<em>events</em>,
                          int <em>n_events</em>,
                          long <em>timeout</em>);</strong></p></div>
<div class="paragraph"><p><strong>int zmq_poller_fd (void *<em>poller</em>, zmq_fd_t *<em>fd</em>);</strong></p></div>
</div>
</div>
<div class="sect1">
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph"><p>The <em>zmq_poller</em>*_ functions provide a mechanism for applications to multiplex
input/output events in a level-triggered fashion over a set of sockets.</p></div>
<div class="paragraph"><p><em>zmq_poller_new</em> and <em>zmq_poller_destroy</em> manage the lifetime of a poller
instance. <em>zmq_poller_new</em> creates and returns a new poller instance, while
<em>zmq_poller_destroy</em> destroys it. A pointer to a valid poller must be passed
as the <em>poller_p</em> argument of <em>zmq_poller_destroy</em>. In particular,
<em>zmq_poller_destroy</em> may not be called multiple times for the same poller
instance. <em>zmq_poller_destroy</em> sets the passed pointer to NULL in case of a
successful execution. <em>zmq_poller_destroy</em> implicitly unregisters all
registered sockets and file descriptors.</p></div>
<div class="paragraph"><p><em>zmq_poller_add</em>, <em>zmq_poller_modify</em> and <em>zmq_poller_remove</em> manage the ØMQ
sockets registered with a poller.</p></div>
<div class="paragraph"><p><em>zmq_poller_add</em> registers a new <em>socket</em> with a given <em>poller</em>. Both <em>poller</em>
and <em>socket</em> must point to valid ØMQ objects. The <em>events</em> parameter specifies
which event types the client wants to subscribe to. It is legal to specify no
events (i.e. 0), and activate them later with <em>zmq_poller_modify</em>.
In addition, <em>user_data</em> may be specified, which is not used by the poller, but
passed back to the caller when an event was signalled in a call to
<em>zmq_poller_wait</em> or <em>zmq_poller_wait_all</em>. <em>user_data</em> may be NULL. If it is
not NULL, it must be a valid pointer. Otherwise, behaviour is undefined.
<em>zmq_poller_add</em> may not be called multiple times for the same socket
(unless <em>zmq_poller_remove</em> has been called for that socket).</p></div>
<div class="paragraph"><p><em>zmq_poller_modify</em> modifies the subscribed events for a socket. It is
legal to specify no events (i.e. 0) to disable events temporarily, and
reactivate them later with another call to <em>zmq_poller_modify</em>.</p></div>
<div class="paragraph"><p><em>zmq_poller_remove</em> removes a socket registration completely.
<em>zmq_poller_remove</em> must be called before a socket is closed with <em>zmq_close</em>.</p></div>
<div class="paragraph"><p>Note that it is not necessary to call <em>zmq_poller_remove</em> for any socket
before calling <em>zmq_poller_destroy</em>.</p></div>
<div class="paragraph"><p>Also note that calling <em>zmq_poller_remove</em> is not equivalent to calling
<em>zmq_poller_modify</em> with no events. <em>zmq_poller_modify</em> does not free resources
associated with the socket registration, and requires that the <em>socket</em>
remains valid.</p></div>
<div class="paragraph"><p><em>zmq_poller_add_fd</em>, <em>zmq_poller_modify_fd</em> and <em>zmq_poller_remove_fd</em> are
analogous to the previous functions but manage regular file descriptiors
registered with a poller. On Windows, these functions can only be used with
WinSock sockets.</p></div>
<div class="paragraph"><p>In the following, ØMQ sockets added with <em>zmq_poller_add</em> and file descriptors
added with <em>zmq_poller_add_fd</em> are referred to as <em>registered objects</em>.</p></div>
<div class="paragraph"><p>The <strong>zmq_poller_event_t</strong> structure is defined as follows:</p></div>
<div class="literalblock">
<div class="content">
<pre><code>typedef struct
{
    void *socket;
#if defined _WIN32
    SOCKET fd;
#else
    int fd;
#endif
    void *user_data;
    short events;
} zmq_poller_event_t;</code></pre>
</div></div>
<div class="paragraph"><p>For each registered object, <em>zmq_poller_wait_all()</em> shall examine the
registered objects for the event(s) currently registered.</p></div>
<div class="paragraph"><p>If none of the registered events have occurred, <em>zmq_poller_wait_all</em> shall
wait <em>timeout</em> milliseconds for an event to occur on any of the registered
objects. If the value of <em>timeout</em> is <code>0</code>, <em>zmq_poller_wait_all</em> shall
return immediately. If the value of <em>timeout</em> is <code>-1</code>, <em>zmq_poller_wait_all</em>
shall block indefinitely until one event has occurred on any of the
registered objects.</p></div>
<div class="paragraph"><p>The <em>events</em> argument <em>zmq_poller_wait_all</em> must be a pointer to an array of
at least <em>n_events</em> elements. Behaviour is undefined if <em>events</em> does not point
to an array of at least <em>n_events</em> elements.</p></div>
<div class="paragraph"><p><em>zmq_poller_wait_all</em> returns at most <em>n_events</em> events. If more than
<em>n_events</em> events were signalled, only an unspecified subset of the signalled
events is returned through <em>events</em>.</p></div>
<div class="paragraph"><p>A caller is advised to ensure that <em>n_events</em> is equal to the number of
registered objects. Otherwise, a livelock situation may result: If more than
<em>n_events</em> registered objects have an active event on each call to
<em>zmq_poller_wait_all</em>, it might happen that the same subset of registered
objects is always returned, and the caller never notices the events on the
others.</p></div>
<div class="paragraph"><p><em>zmq_poller_wait_all</em> returns the number of valid elements. The valid elements
are placed in positions <em>0</em> to <em>n_events - 1</em> in the <em>events</em> array. All
members of a valid element are set to valid values by <em>zmq_poller_wait_all</em>.
The client does therefore not need to initialize the contents of the events
array before a call to <em>zmq_poller_wait_all</em>. It is unspecified whether the
the remaining elements of <em>events</em> are written to by <em>zmq_poller_wait_all</em>.</p></div>
<div class="paragraph"><p><em>zmq_poller_fd</em> queries the file descriptor associated with the zmq_poller,
and stores it in the address pointer to by <em>fd</em>.
The zmq_poller is only guaranteed to have a file descriptor if
at least one thread-safe socket is currently registered.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_event_types">EVENT TYPES</h2>
<div class="sectionbody">
<div class="paragraph"><p>The <em>events</em> parameter of <em>zmq_poller_add</em> and <em>zmq_poller_modify</em>, and the
<em>events</em> member of the zmq_poller_event_t structure are bit masks constructed
by OR&#8217;ing a combination of the following event flags:</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
<strong>ZMQ_POLLIN</strong>
</dt>
<dd>
<p>
For ØMQ sockets, at least one message may be received from the <em>socket</em> without
blocking. For standard sockets this is equivalent to the <em>POLLIN</em> flag of the
<em>poll()</em> system call and generally means that at least one byte of data may be
read from <em>fd</em> without blocking.
</p>
</dd>
<dt class="hdlist1">
<strong>ZMQ_POLLOUT</strong>
</dt>
<dd>
<p>
For ØMQ sockets, at least one message may be sent to the <em>socket</em> without
blocking. For standard sockets this is equivalent to the <em>POLLOUT</em> flag of the
<em>poll()</em> system call and generally means that at least one byte of data may be
written to <em>fd</em> without blocking.
</p>
</dd>
<dt class="hdlist1">
<strong>ZMQ_POLLERR</strong>
</dt>
<dd>
<p>
For ØMQ sockets this flag has no effect on the <em>zmq_poller_add</em> and
<em>zmq_poller_modify</em> functions, and is never set in the
<em>events</em> member of the zmq_poller_event_t structure.
For standard sockets, this flag is passed through <em>zmq_poller_wait_all</em> to the
underlying <em>poll()</em> system call and generally means that some sort of error
condition is present on the socket specified by <em>fd</em>.
</p>
</dd>
<dt class="hdlist1">
<strong>ZMQ_POLLPRI</strong>
</dt>
<dd>
<p>
For ØMQ sockets this flag has no effect on the <em>zmq_poller_add</em> and
<em>zmq_poller_modify</em> functions, and is never set in the
<em>events</em> member of the zmq_poller_event_t structure.
For standard sockets this means there
is urgent data to read. Refer to the POLLPRI flag for more informations.
For a file descriptor, refer to your OS documentation: as an example, GPIO
interrupts are signaled through a POLLPRI event.
This flag has no effect on Windows.
</p>
</dd>
</dl></div>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">The <em>zmq_poller</em>*_ functions may be implemented or emulated using operating
system interfaces other than <em>poll()</em>, and as such may be subject to the limits
of those interfaces in ways not defined in this documentation.</td>
</tr></table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_thread_safety">THREAD SAFETY</h2>
<div class="sectionbody">
<div class="paragraph"><p>Like most other ØMQ objects, a poller is not thread-safe. All operations must
be called from the same thread. Otherwise, behaviour is undefined.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_return_value">RETURN VALUE</h2>
<div class="sectionbody">
<div class="paragraph"><p><em>zmq_poller_new</em> returns a valid pointer to a poller, or NULL in case of a failure.</p></div>
<div class="paragraph"><p>All functions that return an int, return -1 in case of a failure. In that case,
zmq_errno() can be used to query the type of the error as described below.</p></div>
<div class="paragraph"><p><em>zmq_poller_wait_all</em> returns the number of events signalled and returned in
the events array. It never returns 0.</p></div>
<div class="paragraph"><p>All other functions return 0 in case of a successful execution.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_errors">ERRORS</h2>
<div class="sectionbody">
<div class="paragraph"><p>On <em>zmq_poller_new</em>:
<strong>ENOMEM</strong>::
A new poller could not be allocated successfully.</p></div>
<div class="paragraph"><p>On <em>zmq_poller_destroy</em>:
<strong>EFAULT</strong>::
<em>poller_p</em> did not point to a valid poller. Note that passing an invalid pointer (e.g.
pointer to deallocated memory) may cause undefined behaviour (e.g. an access violation).</p></div>
<div class="paragraph"><p>On <em>zmq_poller_add</em>, <em>zmq_poller_modify</em> and <em>zmq_poller_remove</em>:
<strong>EFAULT</strong>::
<em>poller</em> did not point to a valid poller. Note that passing an
invalid pointer (e.g. pointer to deallocated memory) may cause undefined
behaviour (e.g. an access violation).
<strong>ENOTSOCK</strong>::
<em>socket</em> did not point to a valid socket. Note that passing an
invalid pointer (e.g. pointer to deallocated memory) may cause undefined
behaviour (e.g. an access violation).</p></div>
<div class="paragraph"><p>On <em>zmq_poller_add</em>:
<strong>EMFILE</strong>::
TODO</p></div>
<div class="paragraph"><p>On <em>zmq_poller_add</em> or <em>zmq_poller_add_fd</em>:
<strong>ENOMEM</strong>::
Necessary resources could not be allocated.
<strong>EINVAL</strong>::
<em>socket</em> resp. <em>fd</em> was already registered with the poller.</p></div>
<div class="paragraph"><p>On <em>zmq_poller_modify</em>, <em>zmq_poller_modify_fd</em>, <em>zmq_poller_remove</em> or
<em>zmq_poller_remove_fd</em>:
<strong>EINVAL</strong>::
<em>socket</em> resp. <em>fd</em> was not registered with the poller.</p></div>
<div class="paragraph"><p>On <em>zmq_poller_add_fd</em>, <em>zmq_poller_modify_fd</em> and <em>zmq_poller_remove_fd</em>:
<strong>EBADF</strong>*:
The <em>fd</em> specified was the retired fd.</p></div>
<div class="paragraph"><p>On <em>zmq_poller_wait</em> and <em>zmq_poller_wait_all</em>:
<strong>ENOMEM</strong>::
Necessary resources could not be allocated.
<strong>ETERM</strong>::
At least one of the registered objects is a <em>socket</em> whose associated ØMQ
<em>context</em> was terminated.
<strong>EFAULT</strong>::
The provided <em>events</em> was NULL, or <em>poller</em> did not point to a valid poller,
or there are no registered objects and <em>timeout</em> was negative.
<strong>EINTR</strong>::
The operation was interrupted by delivery of a signal before any events were
available.
<strong>EAGAIN</strong>::
No registered event was signalled before the timeout was reached.</p></div>
<div class="paragraph"><p>On _zmq_poller_fd:
<strong>EINVAL</strong>::
The poller has no associated file descriptor.
<strong>EFAULT</strong>::
The provided <em>poller</em> did not point to a valid poller.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_example">EXAMPLE</h2>
<div class="sectionbody">
<div class="listingblock">
<div class="title">Polling indefinitely for input events on both a ØMQ socket and a standard socket.</div>
<div class="content">
<pre><code>void *poller = zmq_poller_new ();

zmq_poller_event_t events [2];
/* First item refers to 0MQ socket 'socket' */
zmq_poller_add (poller, socket, ZMQ_POLLIN, NULL);
/* Second item refers to standard socket 'fd' */
zmq_poller_add_fd (poller, fd, ZMQ_POLLIN, NULL);
/* Poll for events indefinitely */
int rc = zmq_poller_wait_all (items, events, 2, -1);
assert (rc &gt;= 0);
/* Returned events will be stored in 'events' */
zmq_poller_destroy (&amp;poller);</code></pre>
</div></div>
</div>
</div>
<div class="sect1">
<h2 id="_see_also">SEE ALSO</h2>
<div class="sectionbody">
<div class="paragraph"><p><a href="zmq_socket.html">zmq_socket(3)</a>
<a href="zmq_send.html">zmq_send(3)</a>
<a href="zmq_recv.html">zmq_recv(3)</a>
<a href="zmq.html">zmq(7)</a></p></div>
</div>
</div>
<div class="sect1">
<h2 id="_authors">AUTHORS</h2>
<div class="sectionbody">
<div class="paragraph"><p>This page was written by the ØMQ community. To make a change please
read the ØMQ Contribution Policy at <a href="http://www.zeromq.org/docs:contributing">http://www.zeromq.org/docs:contributing</a>.</p></div>
</div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
ØMQ 4.3.2<br />
Last updated 2019-07-08 16:19:27 UTC
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